This revised competing renewal application represents a continuation of our ongoing studies on the mechanisms of leukocyte oxidant and protease interactions in models of acute lung injury in the rat. In our original proposal we hypothesized that oxidant-protease interactions are critical in the development of a model of neutrophil dependent lung injury induced by IgG immune complexes. In fact, we found this to be the case. Oxidants appear responsible for the initiation of the injury but their effects over time appear limited. Rather, serine and metalloproteinases are present in the injured lungs and the addition of the protease inhibitors SLPI and TIMP-2 is markedly protective. In correlative in vitro studies protease- oxidant interactions were found to enhance neutrophil-mediated cytotoxicity and matrix degradation. Thus, the in vivo and in vitro studies suggest that both oxidants and proteases are important in neutrophil-mediated acute tissue injury. Given these previous findings, we propose a series of studies to determine the mechanism(s) of oxidant and protease interactions which result in protease activation and tissue injury. Using this model as well as a macrophage-dependent IgA model we will do a complete time course study to identify proteases in lung and bronchoalveolar lavage, their activity, and susceptibility to regulation by SLPI and TIMP-2. We would predict that activated metalloproteinases from the macrophage are involved in the tissue injury in the IgA model. In fact, this seems to be the case as shown in the preliminary studies. We also believe that it is highly likely that protease activity in vivo is regulated by oxidant and protease interactions. These interactions can occur in one of three possible ways. Oxidants regulate protease activity in vitro by activating metalloproteinases and inactivating the serpin protease inhibitors. If this occurs in vivo then the addition of antioxidants such as catalase should decrease the amount of protease activation. The second possibility is that oxidants and proteases are activated independently but work synergistically to induce lung injury. In this case, the addition of the antioxidants should have no effect on protease activation and the addition of the protease inhibitors no effect on oxidant generation in vivo. Thirdly, proteases in vitro regulate oxidant activity by mechanisms such as priming of the leukocyte oxidant burst and cytokine generation. If this occurs in vivo then the addition of SLPI and TIMP-2 should decrease oxidant levels in the lung. By these methods, we hope to be able to precisely map out how oxidants and proteases interact in neutrophil and macrophage mediated models of acute lung injury.